Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular CMOS image sensors, has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of the image sensor.
Typically each pixel of an image sensor includes a photosensitive element, such as a photodiode, and one or more transistors for reading out the signal from the photosensitive element. For example, a transfer transistor is commonly used in a pixel using a four-transistor (or more) design. The transfer transistor has a transfer gate formed between the photosensitive element and a floating node.
One important issue for both CMOS and CCD image sensors is the potential barrier between the photosensitive element (such as a photodiode) and the transfer gate. The potential barriers and/or wells can prevent full charge transfer and give rise to image artifacts in solid stage imagers. These potential barriers and wells also give rise to image lag issues which occurs when the image signal (electrons) that remain in the photodiode after the image signal has been read out. If the image signal remains in the photodiode, then this image signal can be read out in the next read out as unwanted “old” signal. The old signal from each pixel depends on whether the pixel was focused on a bright or a dark area of the previous frame. The result is that a ghost image of the old scene appears in the new scene photograph or frame.
In image sensors, one method to try to ensure full read out across a transistor is to provide a lateral doping gradient with boron that drives electrons from the photosensitive side, across the transfer gate, to the drain side of the transferred gate (called the floating diffusion). However, it has been found that the addition of a laterally graded doping profile results in unwanted compensation of the photodiode N− implant. While this lateral field can help with transport across the transfer gate, this doping and the photosensitive element/transfer gate interface may actually make the barrier and/or well performance worse. In other words, increasing the boron doping at the transfer gate edge to provide the lateral electrical field increases the barrier at the photosensitive element/transfer gate edge.